1. Field of the Invention
This invention relates to tantalum(V) nitride pigments having improved red pigment colour values. The present invention also relates to the production of the tantalum(V) pigments on the basis of nitriding tantalum(V) oxide with ammonia. The invention finally relates to the use of the tantalum(V) nitride pigments which are improved with regard to colour.
2. Description of the Related Art
Both during use and on disposal, toxicologically questionable constituents may be released from articles coloured or decorated with oxide, sulfide or selenide heavy metal compounds, such as for example cadmium and selenium from cadmium sulfoselenide red pigments. There is accordingly particular interest in using pigments having toxicologically less questionable constituents. Tantalum(V) nitride (Ta.sub.3 N.sub.5) is an innocuous alternative, but the colour values, in particular brightness, of the pigments obtainable using hitherto known processes do not always meet expectations. Hitherto known production processes either give rise to unsatisfactory colour values or entail the use of special tantalum starting compounds.
According to H. Moureau and C. H. Hamblet (J. Amer. Chem. Soc. 59, 33-40 (1937)), the nitride of pentavalent tantalum may be produced by ammonolysing TaCl.sub.5. The nitride obtained in this manner is not pure as it contains oxide constituents. Reaction times are moreover very long. H. Funk and H. Bohland (Z. anorg. allg. Chem. 334, 155-62 (1964)) were able to shorten reaction times by nitriding (NH.sub.4)TaF.sub.6. Due to the volatility of TaF.sub.5 and NH.sub.4 F, yields of Ta.sub.3 N.sub.5 are only low and/or the products contain fluorine. The stated document also disclosed the nitriding of tantalum(V) oxide (Ta.sub.2 O.sub.5) at 800.degree. C. with ammonia; a nitride containing oxide (Ta.sub.3 N.sub.5 (O)) is formed at 800.degree. C., the colour of which was not stated.
On the basis of Funk and Bohland's investigations, G. Brauer and J. R. Weidlein (Angew. Chem. 77, 218-9 (1965)) reexamined the nitriding of pure tantalum pentoxide with purified ammonia: in the presence of titanium chips as oxygen getter, the reaction requires a reaction time of 36 to 120 hours at 860 to 920.degree. C. These reaction times could not be confirmed when these investigations were replicated (c.f. EP-A 0 592 867): colour formation was not complete, i.e. no further colour shift occurred, until a reaction time of 150 hours had elapsed. However, the tantalum(V) nitride obtained in this manner was red-brown and thus of no interest for colouring purposes.
According to the process described in EP-A 0 592 867, it is possible to obtain tantalum(V) nitride having elevated colour intensity and simultaneously shorten the reaction time. In this process, tantalum(V) oxide is replaced by a tantalum(V) oxide hydrate of the formula Ta.sub.2 O.sub.5 aq. having a hydrate content (aq. content) in the range from 14 to 17 wt. %. The duration of the nitriding reaction is shortened by adding fluxes to the tantalum oxide hydrate to be nitrided. The pigment obtained in this manner exhibited a red shift in comparison with prior art products and substantially greater colour intensity than products as were obtainable using tantalum(V) oxide.
One disadvantage of the latter-stated process is that commercially obtainable, low cost tantalum(V) oxide could not be used, but only a specially produced tantalum(V) oxide hydrate. As already explained, one disadvantage of prior processes using tantalum(V) oxide was that they yielded products, the colour values of which prevented the use thereof as a red pigment.